Added support for Short Guids (Sguids)

Added conditional compilation support (SQL_LITE_SUPPORT_SGUID) for Short Guids (Sguids). Due to the way sqlite-net stores Guids using their string representation, it is possible to reduce the storage required from 36 to 22 characters by encoding as Base64 strings. Internally, the Sguid type records the original Guid and converts to and from Base64 strings when required.